This application is directed to an antenna for use in radio frequency identification device (RFID) antennas, and in particular, multi-coil RFID interrogator antennas.
It is known in the art from U.S. Pat. No. 5,012,236 to provide a multi-coil RF antenna for an RFID interrogator. As shown in FIG. 1, the prior art antenna generally indicated as 10 includes a polygonal core 12 formed of iron or plastic. A single transmit antenna 18 is wound about core 12. A receive antenna structure 15 formed of a single wire includes a first receive coil 14 wound about core 12 at a first end of core 12 in a first direction and a second receive coil 16 wound about core 12 at an opposite end thereof wound in a second direction. As a result coil 16 and coil 14 are configured in a differential relationship. In such a relationship a signal received equally at each coil 14, 16 will cancel itself out. A signal which is received with more power at one coil than the other will produce an internal signal to the interrogator which is stronger from one receive coil than the signal produced at the other receive coil, so that after the differential operation, the stronger signal is not entirely cancelled and a signal remains to be processed by the interrogator.
The prior art antenna suffers from several disadvantages. First, the transponder to be monitored is passive and is implanted within an animal. The final position of the implanted transponder cannot be controlled. However, to best be activated, the transponders need a magnetic field to be emitted along the length of the transponder antenna's axis. The magnetic field generated by transmit coil 18 of antenna 10 is aligned almost entirely along the axis of core 12. Therefore, to optimize reading of a transponder, the axis of the transponder must be aligned with the axis of the RFID interrogator antenna. This is not always possible when dealing with implanted live animals which are moving and which conceal (under the skin) the orientation of the transponder.
Another shortcoming of the prior art antenna is that because it is a differential antenna, the receive coils are very sensitive to differential imbalance interference. Furthermore, a differential coil through its action of cancelling out the transmit signal, inherently weakens the signal received by the antenna prior to operation upon the signal by the interrogator. Accordingly, it is desirable to provide an antenna for an interrogator which overcomes the shortcomings of the prior art.